1. Field of the Invention
This invention relates to battery powered vehicles, such as bicycles, which augment the driver""s pedal power with an electric drive system.
2. Description of the Prior Art
It is known in the prior art to provide battery powered bicycles in which an electric motor connected to an electric battery augments a driver""s pedal power. Such prior art systems use various means to overcome problems which arise in their operation. For example, U.S. Pat. No. 5,934,398 is concerned with the problem of xe2x80x9clocked rotorxe2x80x9d which may result in excessively high loads on the transistors. A complex system for detecting a locked state of the motor and calculating the time required for transistor temperatures to reach an upper limit prevents excessively large loads.
The electric bicycle of U.S. Pat. No. 5,971,090 provides torque limiting with an upper duty cycle limit setting device incorporating a table of the relationship between the upper duty limit of a PWM control and motor torque.
In U.S. Pat. No. 3,773,131, electronic control means permits operation of an electric drive system only after the bicycle reaches a predetermined speed as detected by a pickup.
U.S. Pat. No. 5,603,388 discloses as prior art an electrically powered bicycle which includes an induced voltage detection circuit sensing a voltage included in the motor as it turned by pedal power and controlling a PWM circuit to regulate the closure of a motor switch.
Torque, pedal force, or leg power detecting means are shown in the vehicles disclosed in the U.S. Pat. Nos. 5,857,537; 5,664,636; 5,860,487; 5,992,553; 6,006,850; 5,845,727; 6,015,021; 5,505,277; and 5,984,038.
U.S. Pat. No. 3,904,947 shows a vehicle mounted battery charging system in which a comparator monitors voltage for preventing the driver from driving off while the vehicle is plugged into an AC power outlet.
The motorized bicycle shown in U.S. Pat. No. 5,857,537 includes a current sensor in the motor circuit which inputs a value to a CPU which controls a switching element in the motor circuit for duty control.
It is an object of the invention to provide an improved electric drive system for a human-powered vehicle, such as a bicycle. More particularly, it is an object of the invention to provide an electric drive system of this character which effectively performs various functions in a manner which is simple and less complex than drive circuits of the prior art.
According to the invention, a motor drive circuit for a human-powered vehicle, such as a pedal driven bicycle, includes an electric drive motor, energized from a rechargeable battery, and drive means driven by the motor for driving the drive wheel of the vehicle.
The drive motor, which is a D.C. motor, is connected across the battery through a transistor switch and a current sensor resistor. A first comparator monitors the voltage across the sensor resistor. When the voltage across the sensor resistors exceeds a predetermined level as set by a resistor voltage divider, the first comparator changes state and, being connected as an input to a second comparator, pulls down an input to the second comparator, causing the second comparator to change state. The output from the second comparator controls the transistor switch and causes the transistor switch to open the circuit to the drive motor. The transistor switch recloses the circuit to the drive motor when the voltage across the sensor resistor drops below the predetermined level and the voltage level across an RC timing circuit rises to the threshold voltage of the second comparator. The RC timing circuit sets the off time of the transistor switch. By virtue of this arrangement, the duty cycle of the drive motor is established.
When the bicycle is under heavy load, as, for example, when the bicycle is being driven up a steep incline, the drive duty cycle will approach 50%, which indicates that the motor is approaching lock rotor. This situation is monitored by a duty cycle monitoring means including a third comparator. The pulses appearing at the junction between one terminal of the motor and the transistor switch include an A.C. component having negative going portions and positive going portions. A coupling capacitor couples only the A.C. component to an integrating circuit which integrates one of the negative and positive going portions of the A.C. component. When the value of the A.C. component portion integrated reaches a value signifying the approach of 50% duty cycle, the third comparator generates the second control signal to shut off the motor. The integrating circuit includes a diode connected between the coupling capacitor and a reference level for clamping the other portion of the A.C. component to the reference level. A first resistor is connected between the coupling capacitor and an integrating capacitor, which is connected across the input of the third comparator. As the duty cycle approaches 50%, the one portions of the A.C. component are fed to the integrating capacitor; and the charge stored in the integrating capacitor increases until the threshold of the third comparator is reached. This cause the third comparator to provide a signal to open the transistor switch and shut off the motor. A leakage resistor is connected across the diode to provide a leakage path with the first resistor to lower the charge on the integrating capacitor when the time between pulses is large enough (when the duty cycle is not approaching 50% duty cycle).
Because it is undesirable to operate the motor when the bicycle is stationary or operating at a very low speed, the drive circuit includes means to keep the motor from starting until the bicycle speed reaches a predetermined speed, such as approximately four m.p.h. To this end, the circuit includes means to monitor the voltage generated by the motor when it is being turned by operation of the pedals while the motor is not connected across the battery. This means comprises a resistor divider and a monitoring transistor. When the starting switch for the drive circuit is closed, the monitoring transistor monitors the induced voltage generated by the motor. If the induced voltage is less than the predetermined speed, an output from the monitoring transistor inhibits the second comparator from attaining a state which will cause the transistor switch to close the motor circuit. If, however, the induced voltage reaches or exceeds the predetermined speed, the monitoring transistor will permit the second comparator to change state and cause the transistor switch to close the motor circuit.
The system includes a battery charger. A fourth comparator monitors the battery charger and provides an output to the second comparator to inhibit operation of the transistor switch when the charger is energized. In this way, the motor is prevented from running while the motor drive circuit is plugged into the AC line to charge the battery.